1. Field of the Invention
This invention relates to a magnetic recording medium for high-density recording, and more particularly to an improvement of a substrate of a magnetic recording medium used for high-density video recording or the like.
2. Description of the Prior Art
A magnetic recording medium generally comprises a non-magnetic substrate such as polyethylene terephthalate film, polycarbonate film or polyimide film, and a magnetic layer which is provided on the non-magnetic substrate and which consists mainly of ferromagnetic particles and a binder.
In the past, oxide-based fine grains were used as the ferromagnetic grains. Recently, ferromagnetic metal particles are adopted to improve the magnetic recording density and the reproduction output because of their high saturation magnetization and high coercive force.
For the purpose of high-density recording, the magnetic layer must have high surface smoothness to reduce the space loss of electromagnetic transducing characteristics due to the space between a magnetic head and the magnetic tape. Accordingly, it is desired to improve surface smoothness of the magnetic layer by improving the techniques of making the magnetic layer, that is, the dispersing, coating and surface forming techniques for magnetic materials. Further, surface smoothness of the substrate itself is also an important factor for minimizing the space loss because it affects that of the magnetic layer. Particularly, when shorter recording wavelengths are used to improve the recording density, it is advantageous to reduce the thickness of the magnetic layer so as to minimize the influence of the thickness loss. When the thickness of the magnetic layer is reduced, surface smoothness thereof is affected to a larger extent by the surface smoothness of the substrate.
Generally, a plastic film is used as the substrate for the magnetic recording medium. However, surface smoothness of the plastic film can be improved only to a limited extent for the reasons described below.
Namely, if the film formed from a plastic material has good surface smoothness, the frictional resistance between the film and the rollers carrying the film increases, and the film creases or zigzags when it is formed and wound up into a film roll form. Further, the frictional resistance between the film surfaces themselves also increases, so that the film cannot be rolled into the desired cylindrical form but is rolled into a distorted form. Particularly when the tape has a thickness of 25.mu. or less, it is difficult to satisfactorily wind up the tape. In the past this problem has been eliminated by making one or both surfaces of the tape rough. Such a problem arises not only in the manufacture of the plastic film but also in the process for making a magnetic tape by applying a magnetic layer on the plastic film.
In view of the above circumstances, various methods have been proposed to decrease the frictional resistance of a plastic film so as to facilitate the relative movement thereof. For example, it has been proposed in Japanese patent publication No. 43(1968)-23888 to mat the film surfaces by adding a filler to a plastic material used for forming the film. It has also been proposed in Japanese patent publication No. 47(1972)-39929 to mechanically form an uneven film surface by use of a casting drum or the like. An attempt has also been made to decrease the friction coefficient of the film surface by applying a crosslinkable silicone resin onto the film surface as disclosed in Japanese unexamined patent publication No. 52(1977)-18770. In another method, only one surface of a film is matted by extruding a raw plastic material containing a filler. The most popular raw material for plastic films is polyethylene terephthalate (PET).
The above-mentioned problems concerning the film substrate make it impossible to sufficiently improve surface smoothness of a magnetic layer to achieve high-density recording. Accordingly, it has been a general practice to apply a magnetic layer on the side of a film substrate having the latter surface smoothness.